In the distribution of natural gas, the natural gas is conducted from its source in a natural gas supply line at a relatively high pressure. Prior to providing the natural gas to an end user or natural gas customer, a pressure reduction must occur in the natural gas supply line to reduce the pressure of the natural gas to an intermediate pressure or desired level compatible with and suitable for distribution to the end user. To accomplish this function, pressure reduction stations, also known as gate stations or regulating stations, are provided. The pressure reduction station includes pressure reducing valves which reduce the pressure of the natural gas within the natural gas supply line delivering the natural gas to the end user.
As a result of reducing the pressure of the natural gas in the supply line, there is a corresponding decrease in the temperature of the natural gas. The larger the pressure reduction, the larger the decrease in temperature experienced. If the gas temperature is reduced to below freezing, freezing or frosting of the natural gas supply line may occur at the pressure reduction station. Specifically, small amounts of water typically entrained within the natural gas may freeze, causing a blockage of the supply line. This problem is enhanced in colder climates where the natural gas entering the pressure reduction station is already at a reduced or colder temperature.
In order to avoid the difficulties associated with such temperature decreases, the natural gas entering the pressure reduction station is preferably heated prior to the natural gas undergoing the desired pressure reduction. As a result, any pressure reduction of the natural gas is less likely to drop the temperature of the natural gas below freezing. The amount of heat required depends upon, amongst other factors, the temperature of the incoming natural gas to the pressure reduction station and the desired pressure decrease. Heaters which are used to heat natural gas pipelines at pressure reduction stations are commonly referred to as “natural gas line heaters” or “line heaters.”
In addition, in the production of heavy oil, the heavy oil is produced from an underground formation to the surface. Once at surface, the heavy oil and other produced fluids are required to be transported or conveyed to the end user or consumer. However, the fluids produced from the formation generally include a proportion of water and sediment in addition to the heavy oil. As a result, storage or production tanks are typically provided at the well site to permit at least some amount of separation of the fluids produced from the well prior to further distribution or transport.
In particular, the heavy oil and water contained in the produced fluids are preferably permitted to separate such that each may be subsequently transported or conveyed from the well site in an appropriate manner. In addition, sediment from the heavy oil tends to settle or collect at the bottom of the storage tank. The sediment typically includes sand, sludge, scale and other solid, waste or heavier materials. It has been found that this separation process may be facilitated or enhanced by the heating of the produced fluids within the storage tank. Specifically, the produced fluids are typically heated to a temperature sufficient to effectively reduce the viscosity of the heavy oil, but less than the boiling point of the water contained therein.
Similar heating apparatuses or heaters are used in the oil and gas industry in each of the above-noted circumstances. Specifically, heaters are utilized for heating natural gas supply lines at pressure reduction stations, while similar heaters are utilized for heating produced fluids within storage or production tanks. Heaters may also be used generally for heating oil and/or natural gas at various stages of production, refinement, transport and distribution. For example, heaters may be used for heating oil and/or natural gas at wellheads, batteries, pipeline installations etc. Heating of natural gas at a wellhead may prevent freezing of entrained water vapour which may otherwise result due to a pressure reduction at the wellhead. Heating of natural gas upstream of a dehydration facility may facilitate more effective dehydration of the natural gas by preventing water vapour from freezing and dropping out of the gas stream before it can be removed at the dehydration facility.
Conventionally, a line heater consists of a vessel which is filled with an intermediate heating liquid. The natural gas pipeline passes through the vessel and a heat exchanger apparatus communicates with the heating liquid in the vessel to transfer heat to the heating liquid. The heat contained in the heating liquid is then transferred to the natural gas pipeline by conduction. Further, the heat exchanger apparatus which communicates with the heating liquid conventionally consists of a fire tube which is heated by a burner. Similar fire tubes are conventionally used for heating production fluids in a storage tank.
However, in order to avoid the hazards associated with direct heating of the oil or gas, two-phase fluid heaters are alternatively used in the oil and gas industry in each of the above-noted circumstances for indirect heating. Two-phase fluid heaters are utilized as line heaters for heating natural gas supply lines at pressure reduction stations. Similarly, two-phase fluid heaters are utilized for heating produced fluids within storage or production tanks.
Two-phase fluid heaters typically operate using the basic phenomena of evaporation and condensation. In particular, a heat transfer liquid is heated in an evaporator and evaporates to produce a heat transfer vapour. The heat transfer vapour is directed to a condenser. In the condenser, the heat transfer vapour transfers heat to a medium to be heated, with the result that the heat transfer vapour condenses back to the heat transfer liquid. The heat transfer liquid is returned to the evaporator in order to be evaporated again to produce the heat transfer vapour. The cycle is repeated over and over as part of a continuous process.
Various two-phase fluid heaters have been provided for use in the oil and gas industry. However, none have been found to be fully satisfactory.
U.S. Pat. No. 5,947,111 issued Sep. 7, 1999 to Neulander et. al. and Canadian Patent No. 2,262,990 issued May 27, 2003 to Neulander et. al. describe a conventional “thermosyphon heater.” Thermosyphon heaters require gravity for the liquid return. In particular, the heat transfer liquid drains under force of gravity from the condenser to the evaporator. Further, the heat transfer liquid drains within the same conduit that supplies the heat transfer vapour to the condenser, but in the opposite direction.
Canadian Patent No. 1, 264,443 issued Jan. 16, 1990 to Spehar describes a conventional “heat pipe” heater in which the return path for the heat transfer liquid from the condenser to the evaporator consists of a wick. More particularly, the heat pipe heater relies on surface tension pumping in a capillary wick to return the condensate or heat transfer liquid to the evaporator. Thus, the wick enables the heat pipe heater to operate independently of gravity.
U.S. Pat. No. 4,393,663 issued Jul. 19, 1983 to Grunes et. al., Canadian Patent Application No. 2,381,469 published Oct. 12, 2002 by Lange and U.S. Pat. No. 4,660,542 issued Apr. 28, 1987 to Scherer describe another type of two-phase fluid heater which is similar to a thermosyphon heater. In these heaters, the heat transfer liquid returns to the evaporator from the condenser through a conduit which is separate from the conduit that supplies the heat transfer vapour to the condenser, so that the two-phase fluid heater includes a complete “heat driven loop”. Thus, this type of heater is often referred to as a “heat driven loop heater.”
However, in order to function effectively, the heat driven loop heater typically requires a “trap” on the return conduit from the condenser to the evaporator. The “trap” prevents or inhibits the back flow of fluid from the evaporator to the condenser. In other words, heat transfer vapour from the evaporator is restricted from flowing out of the evaporator and to the condenser in a reverse direction through the loop. The trap may be comprised of a restriction or valve in the return conduit or the use of a pressure head at the outlet of the return conduit to the evaporator.
Thus, there remains a need in the industry for an improved heat exchange apparatus for use in transferring heat to a heat sink. Preferably, the improved heat exchange apparatus is for use in transferring heat to a fluid within a natural gas supply line at a pressure reduction station, or alternately, to a fluid within a storage tank.